A 52-year-old man with a 10-year history of hypertension is found on screening to have a left adrenal nodule (2.2 cm) on CT abdomen done for renal artery stenosis evaluation. His BP is 148/92 mmHg despite triple antihypertensive therapy. Laboratory investigations show: morning cortisol 18 µg/dL (normal 10–20), 24-hour urinary free cortisol 95 µg/24 h (normal <50), plasma ACTH 8 pg/mL (normal 10–50), and potassium 3.2 mEq/L. What is the most appropriate next step in management?

Explanation

## Clinical Presentation: Subclinical Cushing Syndrome (SCS) This patient has findings consistent with **subclinical Cushing syndrome (autonomous cortisol-secreting adrenal nodule)**: | Feature | Finding | Interpretation | |---------|---------|----------------| | Adrenal nodule | 2.2 cm (left) | Incidentaloma | | Morning cortisol | 18 µg/dL (upper normal) | Mildly elevated | | 24-h UFC | 95 µg/24 h (elevated) | Autonomous secretion | | Plasma ACTH | 8 pg/mL (suppressed) | Pituitary suppression by cortisol | | Hypokalemia | K⁺ 3.2 mEq/L | Mineralocorticoid excess | | Resistant hypertension | Despite triple therapy | Cortisol-mediated hypertension | **Key Point:** The combination of **suppressed ACTP + elevated UFC + normal/high cortisol** in the setting of an adrenal nodule is pathognomonic for autonomous cortisol secretion. ## Diagnostic Algorithm for Incidentaloma ```mermaid flowchart TD A[Adrenal Incidentaloma]:::outcome --> B{Clinical features of Cushing?}:::decision B -->|Yes| C[Measure 24-h UFC + morning cortisol]:::action B -->|No| C C --> D{UFC elevated OR cortisol high?}:::decision D -->|Yes| E[Low-dose DST 1 mg overnight]:::action D -->|No| F[Reassess annually if <4 cm]:::action E --> G{Cortisol suppresses <1.8 µg/dL?}:::decision G -->|No| H[Autonomous cortisol secretion confirmed]:::outcome G -->|Yes| I[Normal HPA axis]:::outcome H --> J[Adrenalectomy if symptomatic/progressive]:::action I --> F ``` ## Why Low-Dose Dexamethasone Suppression Test (LDDST) Is Next **High-Yield:** The LDDST (1 mg overnight) is the **gold standard confirmatory test** for autonomous cortisol secretion in suspected subclinical Cushing syndrome. ### LDDST Interpretation - **Normal:** Cortisol suppresses to <1.8 µg/dL (intact HPA axis feedback) - **Abnormal:** Cortisol remains >1.8 µg/dL (autonomous secretion, loss of feedback suppression) **Clinical Pearl:** In this patient, ACTH is already suppressed (8 pg/mL), indicating the pituitary is being inhibited by cortisol — the nodule is secreting autonomously. The LDDST will confirm this by showing **non-suppression** of cortisol despite dexamethasone administration. ### Why This Patient Likely Fails LDDST - Suppressed ACTH (8 pg/mL) = pituitary feedback is already broken - Elevated UFC despite normal morning cortisol = autonomous secretion - The nodule will NOT suppress cortisol with dexamethasone [cite:Harrison 21e Ch 375; Endocrine Society Clinical Practice Guideline on Adrenal Incidentaloma]